This invention relates to a method of and apparatus for digital signal processing. The invention will be described with reference to the processing of digital system I PAL colour television signals, to which it is especially applicable, but it is not restricted to application to television signals.
Prior proposals for bit-rate reduction of encoded digital television signals are described in British Pat. Nos. 903,478; 1,029,815 and 1,286,981. These systems all involve discarding a major part of the information before transmission. Thus in the case of British Pat. No. 903,478 either one out of every three or one out of every eight samples only are transmitted, the remaining samples being discarded. In British Pat. No. 1,029,815 one out of every three or one out of every nine samples are transmitted, and in the case of British Pat. No. 1,286,981 either one out of every two or one out of every four or one out of every eight samples are transmitted.
Thus the transmitted sample train is a regular stream of samples at a much reduced rate. The information content of the samples corresponds to a regular pattern along the television line scan. At a receiver the discarded samples cannot be regenerated, but instead the missing samples are replaced by interpolation between the transmitted samples, using some form of interpolating filter.
An error protection system has been described in BBC Research Department Report 1978/19 (M. G. Croll) which is based on proposals in BBC Research Department Report 1977/27 (C. K. P. Clarke), see particularly section 3.2.1, and which makes use of the fact that the system I PAL television signal has a bandwidth which is limited to 5.5 MHz. If such a signal is sampled at three times the colour subcarrier frequency f.sub.sc, then the sample rate is approximately 13.3 MHz, and thus, following Nyquist principles, the digital system can convey frequencies up to half the sampling rate, i.e. 6.65 MHz.
This means that there is a frequency range of between 5.5 MHz and 6.65 MHz which is conveyed by the system but which should in principle contain no frequency components. The error-protection systems of the above Reports effectively operate by looking for information in this frequency range, and assuming that any such information is caused by errors introduced into the signal during processing or transmission. With the aid of an estimator based on a high-pass symmetrical digital transversal filter, it is shown to be possible to determine which bits of the digital signal are in error, and to determine the sense or polarity of the error. This enables errors in a binary television signal to be detected and corrected.
Reference should be made to the above-mentioned Reports for further details of the error-protection system and its theory of operation.